Well I typed a bit and lost it so this will be shorter.
Why is there change water being used from the store?
Where did the black start on the fish. Tank details and stocking? The black could be chemical burns - pick up the API freshwater master test kit to check ammonia and ph. Are you using a water conditioner?
If you have methylene blue, do a single half-dose into tank. This will help with low-level infection cleanup + breathing.
Methylene Blue | Kordon
Did you introduce anything to the tank a month ago?
Hope your fish pulls through.